In an electronic imaging apparatus, such as a portable projector, light from each of a number of different spectral bands, conventionally Red, Green, and Blue (RGB), is combined in order to provide a color image. For large-scale devices, separate modulation and projection optics may be used for each spectral band, converging the component color images to form a composite, multicolor image onto a display screen or other display surface. For more portable projection apparatus, however, a coaxial mixing that combines the colored light from each channel onto a common optical axis is often more desirable. The coaxial mixing arrangement uses the same projection optics for each color channel and allows optical component functions and light modulation to be shared for each color channel where possible, to conserve space and cost.
Earlier electronic imaging apparatus employed lamps and other polychromatic light sources to provide the colored light in each color channel. A number of different types of light-mixing systems were developed to support the light combining function for these earlier systems, including complex prism arrangements adapted from color television camera optics, for example. With the advent of solid-state light sources, such as Light Emitting Diodes (LEDs) and lasers, it became possible to reduce some of the size and cost of color mixing components, as well as improving color gamut, optical efficiency, and overall performance of the imaging device. For color combination, various arrangements of composite prisms and dichroic coatings were developed for use in projectors and similar imaging apparatus. Among the more familiar solutions used with earlier electronic imaging systems are X-cubes or X-prisms, as shown in FIG. 1, and related dichroic optical elements, and Philips prisms. Non-prism solutions include sets of angled dichroic plates, such as those as proposed in U.S. Pat. No. 6,676,260 (Cobb et al.) entitled “Projection Apparatus Using Spatial Light Modulator with Relay Lens and Dichroic Combiner.”
Referring to FIG. 1A, an X-cube 10 is a composite prism, formed from four separate prism elements 10a, 10b, 10c, and 10d, as shown in inset E1, that are coated and then glued together to form a single color combiner component. The X-cube combines light from three solid-state light sources 14r, 14g, and 14b, such as laser diodes, emitting red, green, and blue light respectively. As assembled, X-cube 10 has two inner crossed dichroic interfaces 12a and 12b that are treated to selectively reflect and transmit different wavelengths. Dichroic interface 12a reflects blue wavelengths and transmits green and red wavelengths. Dichroic interface 12b, contiguous to dichroic interface 12a so that the two interfaces intersect along a line through the center of the X-cube, reflects red wavelengths and transmits green and blue wavelengths. The line of intersection of the dichroic surfaces is orthogonal to the plane of the drawing as X-cube 10 is represented in FIG. 1A. The colors are combined onto an optical axis OA, shown with separate color paths for clarity in FIG. 1, but coaxial in practice.
The Philips prism 70, shown in FIG. 1B, is a more complex composite prism that is used for combining colors. Philips prism 70 is formed from three separate prisms, prism elements 70a, 70b, and 70c, and includes an air gap 72. An arrangement of dichroic surfaces at oblique angles directs light from solid-state light sources 14r, 14g, and 14b onto optical axis OA.
Dichroic surfaces are formed from stacked layers of ultra-thin coatings of various dielectric materials and can be formulated to provide selective reflection and transmission for light of various wavelengths. In the X-cube and Philips prism devices, and other related spectral combiners or separators, various types of dichroic coatings provide the color-selective mechanism that allows light to be spectrally re-combined or separated in a highly controlled manner.
Small, hand-held projectors and various types of embedded or accessory projectors typically use an arrangement of dichroic surfaces for mixing red, green, and blue laser light onto a single optical axis. These devices then rapidly scan the resulting light onto a display surface. To minimize battery usage and heat generation, these devices form each pixel by directly modulating each of the lasers, thereby producing only the light that is used to form the image itself. These projectors form an image by generating successive scanned lines of pixels that are then directed to the display surface.
While hand-held projection devices achieve good results using conventional color combination techniques, however, a number of problems remain. One problem with conventional color combination using dichroic surfaces relates to incident angles. Dichroic coatings reflect and transmit light as a function of incident angle and wavelength. As the incident angle varies, the wavelength of light that is transmitted or reflected also changes. For light that is incident at low angles, that is, at angles close to normal, this variation in response over a small range of angles, relative to wavelength, can be very low or negligible. For light incident at larger angles, however, variation in response over a range of angles can be pronounced, compromising dichroic coating performance. These coatings work best at small incident angles, relative to a normal, and it can be expensive and difficult to design and fabricate a dichroic coating that gives uniform results with incident light at larger angles or over a wide range of angles. Where a dichroic coating must accept light over a large range of angles, perceptible color non-uniformities can easily result.
Other drawbacks of existing color combiners relate to the number of surfaces upon which the light is incident, whether it reflects from or transmits through the surface. Each additional surface represents some efficiency loss and reduced brightness. In addition, each interaction of the light reflecting from or transmitting through a dichroic surface causes some loss, due to imperfect dichroic performance. Due to some amount of light leakage, contrast is also compromised each time the light is incident on a surface, whether or not the surface is coated. With the arrangement of X-cube 10 in FIG. 1A, for example, none of the light beams is incident on fewer than three surfaces. With the arrangement of Philips prism 70 in FIG. 1B, none of the light beams is incident on fewer than four surfaces.
Another consideration relates to reflection and transmission of polarized light. As incident angles for light on the dichroic surface become larger, differences in polarization handling for reflected light become increasingly more pronounced. Moreover, with incident polarized light from multiple laser sources, unique polarization characteristics of the laser diodes themselves must be taken into account. One or more phase retarders can be added in various color channels, but add cost and complexity. The task of designing and fabricating dichroic coatings that allow combination of laser light of different wavelengths but the same polarization presents a considerable and costly challenge.
A further problem relates to dimensional characteristics of the laser beams themselves. The solid state laser beam, considered in cross-section, is typically anamorphic in shape, more elliptical rather than circular, with distortion relative to its orthogonal axes. As a further complication, the amount of anamorphic distortion of the beam varies from one color to the next, making it difficult to form a uniformly sized pixel from three component colors and degrading color quality.
Cost is another concern. As FIGS. 1A and 1B show, conventional solutions for color mixing are characterized by complex arrangements of dichroic surfaces and associated prism elements. A number of fabrication and precision assembly operations are required for their implementation. In conventional manufacture, two or more separate prism elements are formed, then dichroic coatings are applied to one or more outer surfaces. Once coatings are formed, the two or more prisms are then glued together, or are otherwise mounted together in a precise geometric arrangement. As a result of this assembly, one or more dichroic coatings are typically inside the color combiner, surrounded by glass or other transparent material that forms the separate prism elements. Because of coatings tolerances, slight misalignment or angular inaccuracy in prism positioning, unwanted air gaps or imperfections, and other factors, color mixing performance can be compromised.
There is, then, a need for a color mixing solution for solid-state light sources that reduces cost over conventional approaches and offers a reduced number of incident surfaces, reduced incident angles, improved polarization response, and improved beam-shaping.